Brussels was transformed into a fortress Saturday — with subways closed and heavily-armed cops and soldiers patrolling the streets — after intelligence officials caught wind of a “serious and imminent threat” of Paris-style attacks.

The decision to lock down the Belgian capital was “based on quite precise information about the risk of an attack like the one that happened in Paris,” Prime Minister Charles Michel said.

Speaking at a Saturday news conference, Michel added authorities fear that “several individuals with arms and explosives could launch an attack, perhaps even in several places at the same time.”

Officials urged residents to avoid rail stations and airports, shopping malls, concerts and other public events that draw large crowds.

The source of the threat wasn’t released.

Belgian soldiers stand guard outside the Radisson Blu hotel in central Brussels, November 21, 2015, after security was tightened in Belgium following the fatal attacks in Paris.

(YOUSSEF BOUDLAL/REUTERS)

The security clampdown came hours after news broke of four new arrests linked to the Paris attack probe — one in Brussels and three in Turkey.

One suspect was taken into custody after investigators searching a suburban Brussels home discovered weapons but no explosives, authorities said.

No other details were released but the suburb of Molenbeek, where the man was detained, is a known base for some of the Paris terrorists.

One of the three men taken into custody in Turkey was believed to have scouted areas in Paris that were targeted in the attacks.

The 26-year-old suspect — Ahmet Dahmani, a Belgian national of Moroccan descent, was arrested at a hotel in Antalya, CNN reported.

Turkish police detain a man in Antalya, Turkey, late Friday. Turkey's state-run news agency says authorities have detained three suspected Islamic State militants, including a 26-year-old Belgian of Moroccan descent.

(AP)

The 26-year-old is believed to have scouted out the locations for the Novfember 13 attacks in Paris.

(IHLAS NEWS AGENCY/EPA)

Three men were arrested in Turkey late Friday night, one of whom is a 26-year-old Belgian man of Moroccan descent with ties to ISIS who scouted the area before the Paris attacks.

Two other suspects — Syrian nationals Ahmet Tahir, 29, and Mohammed Verd, 23 — were collared after they traveled from Syria to meet Dahmani, authorities said. The pair were apparently planning to transport him to Islamic State-controlled areas in Syria, according to CNN.

In Brussels, cops and soldiers carrying heavy weapons were guarding major intersections throughout the city.

Most shops were closed, but Brussels Airport remained open amid ramped-up security. “We urge the public not to give in to panic, to stay calm,” Michel said after Belgium’s special security cabinet held an emergency meeting.

The U.S. Embassy in Belgium urged Americans in the country to “shelter in place and remain at home.”

“If you must go out, avoid large crowds,” the embassy added in a statement on its website.

By nightfall, the city center was largely deserted save clusters of tourists. Most shops had closed early at the urging of the mayor.

“Of course I’m afraid. We all are,” Linda Faraj, who was selling waffles out of one of the few open stores near the Grand Plaza, told the Washington Post.

Eight days after Paris saw its deadliest violence in decades, investigators around the world were locked in a feverish effort to uncover how the mostly French and Belgian extremists with ISIS links pulled off the synchronized slaughter.

At least one suspect, Salah Abdeslam, was still at large after crossing into Belgium the morning after the attacks.

Two of Abdeslam’s friends told ABC News he was hiding out in the Brussels area and, in a desperate Skype call earlier in the week, had asked them to help him get back to Syria.

Belgian soldiers and a police officer patrol in central Brussels.

(YOUSSEF BOUDLAL/REUTERS)

The subways in Brussels were all closed on Saturday, due to the threat of a terrorist attack.

(JOHN THYS/AFP/Getty Images)

But Paris authorities conceded Saturday that they had no idea where Abdeslam may be hiding.

The suspected militant was last seen in a car pulled over by police near the Belgian border.

Abdeslam was let go because his role in the attacks was not yet known.

The two Belgian men riding in the car with him have since been detained. A lawyer for one of them told French media her client said Abdeslam appeared agitated and “may have been ready to blow himself up.”